#include <stdio.h>

int main(){
    long long A, B, C;
    long long X = 0;
    scanf("%lld %lld %lld", &A, &B, &C);
    A *= 60;
    C *= 3600;
    if( A <= B ){
        X = -1;
    }else{
        X = C / (A-B) ;
        if( C % (A-B) != 0 ) X++;
    }
    printf("%lld", X);
    return 0;
}