#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cmath>

#define rep(i,n) for(i=0; i<n; ++i)
#define in(a) cin >> a
#define out(a,b) cout << a << b
using namespace std;
using lint = long long;

int main(void){
    int i, j, n, m, k, cnt = 0;
    in(k);
    in(n);
    in(m);
    vector<int> f(m);
    rep(i,m)    in(f[i]);
    int mame = k*n;
    int sum = 0;
    rep(i,m)    sum+=f[i];
    int ans = -1;
    mame-=sum;
    if(mame>=0) ans = mame;
    out(ans,endl);

    return 0;
}