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import java.io.IOException;
import java.io.InputStream;
import java.util.Arrays;


public class kug {
 
	public static void main(String[] args) throws IOException {
		Timer timer = new Timer();
		
		final InputStream inputStream = System.in;
		final SimpleNumber simpleNumber = new SimplePositiveInteger();
		final SimpleStringParser simpleStringParser = new SimpleStringParser(100000, inputStream);
		
		int n = simpleStringParser.parseLastInt(simpleNumber, true);
		int[][] array = new int[n][];
		for(int i = 0;i < n;i++) {
			int length = n - i - 1;
			array[i] = new int[length + 1];
			for(int j = 0;j < length;j++) {
				array[i][j] = simpleStringParser.parseInt(simpleNumber);
			}
			array[i][length] = simpleStringParser.parseLastInt(simpleNumber, length > 0);
		}
		
		long sum = 0;
		for(int i = 0;i < n;i++) {
			int min = Integer.MAX_VALUE;
			for(int j = i, k = 0;j > 0;j--, k++) {
				int value = array[k][j];
				if(min > value) {
					min = value;
				}
			}
			
			int length = array[i].length;
			for(int j = 0;j < length;j++) {
				int value = array[i][j];
				if(min > value) {
					min = value;
				}
			}
			
			sum += min;
		}
		
		System.out.println(sum);
		timer.printTimeInMillis();
	}
	
	static class SimpleStringParser {
		int index = 0;
		final byte[] array;
		int bytesReaded;
		final InputStream inputStream;
		
		public SimpleStringParser(final int arraySize, final InputStream inputStream) throws IOException {
			array = new byte[arraySize];
			this.inputStream = inputStream;
			bytesReaded = inputStream.read(array);
		}
		
		protected final void nextChunkOfData() throws IOException {
			bytesReaded = inputStream.read(array);
			index = 0;
		}
		
		protected final boolean nextChunkOfDataRequired() {
			return bytesReaded <= index;
		}
		
		public int parseInt(final SimpleNumber simpleNumber) throws IOException {
			final int integer = simpleNumber.parseInt(this);
			++index;
			return integer;
		}
		
		public int parseLastInt(final SimpleNumber simpleNumber, final boolean hasMore) throws IOException {
			return simpleNumber.parseLastInt(this, hasMore);
		}
	}
	
	static abstract class SimpleNumber {
		protected static final String LINE_SEPARATOR = System.getProperty("line.separator");
		protected static final char BEGIN_OF_LINE_SEPARATOR = LINE_SEPARATOR.charAt(0);
		protected static final int ZERO = '0';
		protected static final int SPACE = ' ';
		
		public int parseInt(SimpleStringParser simpleStringParser) throws IOException {throw new UnsupportedOperationException();}
		public int parseLastInt(SimpleStringParser simpleStringParser, boolean hasMore) throws IOException {throw new UnsupportedOperationException();}
	}
	
	static class SimplePositiveInteger extends SimpleNumber {
		@Override
		public int parseInt(final SimpleStringParser simpleStringParser) throws IOException {
			if(simpleStringParser.nextChunkOfDataRequired()) {
				simpleStringParser.nextChunkOfData();
			}
			if(simpleStringParser.array[simpleStringParser.index] == SPACE) {
				++simpleStringParser.index;
			}
			int integer = simpleStringParser.array[simpleStringParser.index++] - ZERO;
			int value;
			if(simpleStringParser.nextChunkOfDataRequired()) {
				simpleStringParser.nextChunkOfData();
			}
			while((value = simpleStringParser.array[simpleStringParser.index]) != SPACE) {
				integer *= 10;
				integer += value - ZERO;
				++simpleStringParser.index;
				if(simpleStringParser.nextChunkOfDataRequired()) {
					simpleStringParser.nextChunkOfData();
				}
			}
			return integer;
		}
		
		@Override
		public int parseLastInt(final SimpleStringParser simpleStringParser, final boolean hasMore) throws IOException {
			if(simpleStringParser.nextChunkOfDataRequired()) {
				simpleStringParser.nextChunkOfData();
			}
			int integer = simpleStringParser.array[simpleStringParser.index++] - ZERO;
			int value;
			if(simpleStringParser.nextChunkOfDataRequired()) {
				simpleStringParser.nextChunkOfData();
			}
			while((value = simpleStringParser.array[simpleStringParser.index]) != BEGIN_OF_LINE_SEPARATOR) {
				integer *= 10;
				integer += value - ZERO;
				++simpleStringParser.index;
				if(simpleStringParser.nextChunkOfDataRequired()) {
					simpleStringParser.nextChunkOfData();
				}
			}
			if(hasMore) {
				simpleStringParser.index += LINE_SEPARATOR.length();
				if(simpleStringParser.nextChunkOfDataRequired()) {
					simpleStringParser.nextChunkOfData();
					for(int i = 0;i < LINE_SEPARATOR.length();++i) {
						if(simpleStringParser.array[i] == LINE_SEPARATOR.charAt(i)) {
							++simpleStringParser.index;
						}
					}
				}
			}
			return integer;
		}
	}
	
	static public class Timer {

		private long time;
		
		public Timer() {
			startNextMeasure();
		}
		
		public void startNextMeasure() {
			time = System.nanoTime();
		}
		
		public void printTimeInMillis() {
			System.err.println(getTitmeInNanos() / 1000000);
		}
		
		public void printTimeInNanos() {
			System.err.println(getTitmeInNanos());
		}
		
		public void printTimeInMicros() {
			System.err.println(getTitmeInNanos() / 1000);
		}
		
		public void printTimeInSeconds() {
			System.err.println(getTitmeInNanos() / 1000000000);
		}
		
		private long getTitmeInNanos() {
			return (System.nanoTime() - time);
		}
	}
	
}