#include <iostream>
using namespace std;

int main(void){
    int K, N, F, a, b=0;

    cin >> K >> N >> F;
    N *= K;
    while(F--){
        cin >> a;
        b += a;
        if(b>N){
            b = N + 1;
            break;
        }
    }
    cout << N-b << endl;

    return 0;
}