#include <stdio.h>
int main(void){
    int K, N, F, A, sum = 0;
    scanf("%d%d%d",&K, &N, &F);
    while(F != 0){
        scanf("%d", &A);
        sum += A;
        F--;
    }
    if(K * N >= sum){
        printf("%d\n", K * N - sum);
    }
    else{
        printf("%d\n", -1);
    }
    return 0;
}