C++ :

#include <cstdio>
int main(){
    int M, N; if(scanf("%d %d", &M, &N) != 2) return 0;
    int d = N - M;
    if(d <= 0){ printf("0\n"); return 0; }
    if(d == 1){ printf("1\n"); return 0; }
    unsigned long long a = 1, b = 2;      /* f(1), f(2) */
    for(int i = 3; i <= d; i++){
        unsigned long long c = a + b;
        a = b; b = c;
    }
    printf("%llu\n", b);
    return 0;
}