#include<bits/stdc++.h>
using namespace std;

int main()
{
int K,N,F;scanf("%d %d %d\n",&K,&N,&F);
int mame=K*N;vector<int> A(F);
for(int i=0;i<F;i++)scanf("%d ",&A[i]);
for(int i=0;i<A.size();i++)mame-=A[i];
if(mame>=0)printf("%d\n",mame);
else printf("-1\n");
}