#include<iostream>
#include<iomanip>
#include<algorithm>
#include<vector>
#include<string>
#include<math.h>

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,n) for(int i=0;i<(n);i++)
#define ALL(i) (i).begin(),(i).end()
#define rALL(i) (i).rbegin(),(i).rend()
#define debug(x) cout<<#x<<": "<<x<<endl
#define pi M_PI

typedef long long ll;
typedef unsigned long long ull;

using namespace std;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
/*-------- code --------*/
    int k, n, f;
    int t;
    int a[100];

    cin >> k >> n >> f;
    FOR(i,0,f) cin >> a[i];
    t = k * n;
    FOR(i,0,f){
        t -= a[i];
        if(t < 0){
            cout << -1 << endl;
            return 0;
        }
    }
    cout << t << endl;
}