I went for a non-array approach. Feedback much appreciated :)

for(int j = 0; j < numInputs; j++){
    for(int k = 0; k < numInputs; k++){
        cin >> curInput;
        if(j == k){
            leftD += curInput;
        }
        if(j+k == (numInputs-1)){
            rightD += curInput;
        }
    }
}
ans = abs(leftD-rightD);