#include<bits/stdc++.h>
using namespace std;
#define NUM 11
int main(void){
    int K, N, F; cin >> K >> N >> F;
    int cnt = K * N;
    int A;
    for(int i=0; i<F; i++){
        cin >> A;
        cnt -= A;
    }
    if(cnt < 0){
        cout << -1 << endl;
    }else{
        cout << cnt << endl;
    }
    return 0;
}