#include <iostream>

using namespace std;

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    int A[f];
    for(int i = 0; i < f; i++){
        cin >> A[i];
    }
    int sum = 0;
    for(int i = 0; i < sizeof(A)/sizeof(A[0]); i++){
        sum += A[i];
    }
    int result = k * n - sum;
    if(result < 0){
        cout << -1 << endl;
    }else{
        cout << result << endl;
    }
    return 0;
}