Codeforces Round #491 (Div. 2)A. If at first you don't succeed...

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A. If at first you don’t succeed…

1. 题意：
   给你A,B,C集合的大小，求D集合的大小
2. 链接：http://codeforces.com/contest/991/problem/A
3. AC code:

#include<iostream>
using namespace std;
int main(){
    int a,b,c,n;cin>>a>>b>>c>>n;
    int ans=n-(a-c+b);
    if(c<=a&&c<=b){
        if(ans<1)cout<<-1<<endl;
        else cout<<ans<<endl;
    }
    else cout<<-1<<endl;    
}