#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <cstring>

typedef long long ll;
using namespace std;

#define mod 1000000007
#define INF 1000000000 //(int)1e9
#define LLINF 2000000000000000000 //(ll)2e18
#define PI  3.1415926536

#define SIZE 1000

int main(){
    int k,n,f,a;
    int ans = 0;
    
    scanf("%d%d%d",&k,&n,&f);
    
    ans = k*n;
    
    for(int i=0;i<f;i++){
        scanf("%d",&a);
        ans-=a;
    }
    
    
    
    printf("%d\n",max(ans,-1));
    
    return 0;
}