#include <stdio.h>
#include<math.h>

int main(void){
    int i,n,m,count=0;
    if(scanf("%d %d",&n,&m)!=0){
    int* a=new int[m];
    for(i=0;i<m;i++) {if(scanf("%d",&a[i])!=0) count+=a[i];  }
    if(count<=m)printf("%d\n",m-count);
    else printf("-1");
    }}