Get first col in matrix

I have simple program that gets the sum of main and anti diagonals of a matrix. And then it get the sum of first and last column of a matrix.For example:

1 2 3
4 5 6  --> The matrix
7 8 9

md = 1 + 5 + 9 = 15
ad = 7 + 5 + 3 = 15
lastCol = 3 + 6 + 9 = 18
firstCol = 1 + 4 + 7 = 12

How can i get the sum of the firstCol of a square matrix ?
Here is my code:

int main(){
    int n;
        scanf("%d",&n);
        int i,j,a[n][n],firstCol=0,lastCol=0,md=0,ad=0;
        for(i = 0;i <n;i++){
            for(j=0;j<n;j++){
                scanf("%d",&a[i][j]);
            }
        }
        for(i = 0;i <n;i++){
            for(j=0;j<n;j++){
                if(i==j){
                    md+=a[i][j];
                }
                if(i+j==n-1){
                    ad+=a[i][j];
                }
            }
        }
        for(i=0;i<n;i++){
                lastCol+=a[i][n-1];
        }
}

>Solution :

for (i = 0; i < n; i ++) {
    firstCol += a[i][0];
}