/*
Task: Problem 7.05
Date: Dec 24, 2020
Author: aLittleLove (Minh Vu)
*/
 
#include<bits/stdc++.h>
 
using namespace std;
 
int main()
{
    ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
    //freopen("input.txt","r",stdin);
    int n; cin >> n >> n;
    double sum = 0;
    for (int i=0; i<n; i++)
    {
        for (int j=0; j<n; j++)
        {
            double x; cin >> x;
            if (i==j) sum += x;
        }
    }
    cout << sum;
    return 0;
}

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5 5
0 6 8 9 2
2 5 5 8 3
3 1 0 2 8
8 9 3 4 9
1 3 7 6 2