#include<iostream>
#include<sstream>
#include<vector>

int main(){
    int K,N,F;
    std::cin >> K >> N >> F;
    int beannum = K*N;
    for(int i=0;i<F;i++){
        int theAge;
        std::cin >> theAge;
        beannum -= theAge;
    }
    if(beannum<0){
        beannum = -1;
    }
    std::cout << beannum << std::endl;
}