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

#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#define PB push_back
#define ALL(a) (a).begin(),(a).end()
#define PII pair<int,int>
#define VS vector<string>
#define VI vector<int>
#define V(type) vector<type>
#define DEBUG(x) cout<<#x<<": "<<x<<endl


using namespace std;

int main(int argc, const char * argv[]) {
    int K, N, F, bn; cin >> K >> N >> F;

    bn = 0;
    REP(i,F){
        int temp_i; cin >> temp_i;
        bn += temp_i;
    }
    
    int result = (( K * N - bn ) >= 0 ) ? ( K * N - bn ) : -1;
    cout << result << '\n';
    


    return 0;
}