#include <fstream>
using namespace std;
ifstream fin("coada2.in");
ofstream fout("coada2.out");
int main()
{
    long long x,y,z;
    fin>>x>>y>>z;
    if(z==0 || y==0 || x+y+z==0 || x>=z || x>=y) fout<<-1;
    else fout<<y+z-x;
    return 0;
}