#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

int main() {
    
    int K,N,F;
    cin >> K >> N >> F;
    
    int A = 0;
    for ( int i = 0; i < F; i++ ) {
        int a;
        cin >> a;
        A += a;
    }
    
    cout << ( K*N - A >= 0 ?  K*N - A : -1 ) << endl;
    
    return 0;
}