#include <iostream>
using namespace std;

int main() {
    int leavingCars, minutes;
    int carPerMinute[100];
    int totalCars = 0;

    cin >> leavingCars >> minutes;
    int carsLeavingPerMinute = leavingCars * minutes;

    for(int index = 1; index <= minutes; ++index) {
        cin >> carPerMinute[index];
        totalCars += carPerMinute[index];
    }

    cout << totalCars - carsLeavingPerMinute;

    return 0;
}