Forest density

#include <iostream>
#include <vector>


int main(int argc, char** argv) {
    int n, q;
    std::cin >> n >> q;

    std::vector<std::vector<long>> sum_until(n+1);
    for (int i=0; i<n; i++) {
        char c;
        int sum = 0;
        for (int j=0; j<n; j++) {
            std::cin >> c;
            sum_until.at(i).at(j+1) = sum + 1 ? (c=='.') : 0;
        }
    }

    for (int i=0; i<q; i++) {
        int a, b, c, d;
        std::cin >> a >> b >> c >> d;

        int result = sum_until.at(a).at(b) - sum_until.at(c).at(b) - sum_until.at(a).at(d) + sum_until.at(c).at(d);

        std::cout << result << std::endl;
    }

}

Test 1

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 1) >= this->size() (which is 0)

Test 2

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 1) >= this->size() (which is 0)

Test 3

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 1) >= this->size() (which is 0)